Light emitting devices, such as light emitting diodes (LEDs) for example, are often packaged within surface mounted device (SMD) housings. These housings are often made of plastic and are referred to as plastic leaded chip carriers (PLCCs). SMD housings can typically feature an LED connected to multiple metal leads. Portions of the leads may be molded within a plastic body, while other portions may protrude and extend outside of the plastic body. The molded plastic body can define a reflector for enhanced light emission and can be coated with an encapsulant containing a phosphor, such as yttrium aluminum garnet (YAG) for obtaining light having a desired wavelength spectrum. The body of the SMD housing can also comprise a ceramic material. The metal leads of the leadframe package serve to as a channel for supplying the LED with electrical power and, at the same time, may act to draw heat away from the LED chip.
Heat is generated by the LED when power is applied to the LED to produce light. The portion of the leads that can extend out from the package body can connect to circuits external to the leadframe package, for example those on a printed circuit board (PCB). Some of the heat generated by the LED may be dissipated by the plastic package body; however, it is desirable for most of the heat to be drawn away from the LED via the metal components, or other elements of high thermal conductivity. To increase the heat dissipating capacity of an LED package, a heat transfer material or substrate such as a heat slug may be introduced into the package. Standard soldering processes such as lead-free reflow are used for assembly of the LED packages to external sources, such as PCBs. Once soldered, the heat slug can draw heat from the LED chip to an external source, thus increasing the heat dissipating capacity of the LED package. However, conventional package designs utilize designs wherein the external surface of the heat slug is flush with an external surface of the metal leads on a side of the package in which the surfaces will become attached to the PCB. To be adequate, the metal leads need only establish an electrical contact with the PCB. However, adequate contact between the PCB and heat sink is more difficult because in order to ensure adequate thermal transfer, ideally the entire bottom surface of the heat sink needs wetted by the solder to minimize voids. Current LED package designs can result in the surface of the heat sink being inadequately wetted, thus inadequately soldered to the PCB and thereby decreasing both reliability and heat dissipation of the LED package. If not adequately wetted, voids can exist between the bottom surface of the heat transfer material and the PCB, thus resulting in poor heat transfer as well as heat dissipation problems.
Consequently, there remains a need for improved light emitting device packages that overcome or alleviate shortcomings of prior art light emitting device packages.